SATB1, a protein expressed predominantly in thymocytes, binds to double-stranded genomic sequences with high base-unpairing propensity referred to as base unpairing regions (BURs). Ablation of SATB 1 by gene targeting results in temporal and spatial misexpression of numerous genes and arrested T-cell development, suggesting that SATB 1 is a cell-type specific global gene regulator. SATB1 has a unique cage-like nuclear distribution, called the 'SATB1 network,' circumscribing heterochromatin in thymocytes, which largely resists salt extraction. The SATB 1network was shown to fold chromatin by anchoring BUR sequences onto itself, making these sequences the bases of chromatin loop domains. Furthermore, SATB1 recruits chromatin remodeling/modifying complexes to the BURs at chromatin bases and regulates chromatin structure over long distances. In SATB 1-null mice, the CD3-CD4-CD8- triple negative (TN) cells suddenly decrease within 8-10 days after birth, and the total thymocyte cellularity decreases by 2 weeks of age, followed by massive thymocyte apoptosis after 2.5 weeks. To investigate the mechanisms underlying these phenotypes, we will study whether there are any age-dependent defects in lymphoid progenitor cell production in SATB1 null mice (Specific Aim 1). Because SATB1 null thymocytes prior to two weeks of age are also hypersensitive to DNA damaging agents, we will study the potential role of the SATB 1 network on DNA repair (Specific Aim 2). Early during caspase-dependent apoptosis, SATB 1 is cleaved by caspase 6 and rapidly dissociates from chromatin. We made a mutation at this cleavage site so that SATB 1 resists cleavage during apoptosis. We propose to establish the CD2 enhancer/promoter driven-mutated SATB 1 and wild-type control transgenic mouse lines to examine the effects of this mutation on thymocyte apoptosis. We will also test whether the SATB 1 cleavage, which normally occurs in apoptosis, is also important in other biological processes, such as T cell differentiation (Specific Aim 3).